Interpretive Summary: The elms are one of the most important tree crops for the $4.7-billion per year nursery industry. The commercial importance of the genus centers on the American elm. Once decimated by Dutch Elm Disease, the recent introduction of commercial American elms that are resistant to the disease has led to a resurgence in horticultural use of the species, but this resurgence is currently based on very few cultivars, representing a perilously narrow genetic base. Current scientific references state that American elm is tetraploid (that is, genetically it has four copies of each chromosome). Ploidy level (number of copies of each chromosome per cell) were estimated for American elm trees from across its native range in eastern North America. Two ploidy levels were found. Tetraploid trees are common and widespread, but so are diploid trees (with two copies of each chromosome), and both ploidy levels occur together in some areas. Understanding the genetic relationships among the different ploidy levels and their geographic distribution and genetic relationship to one another will allow more efficient surveying for disease-resistant American elms, which will contribute towards developing additional disease tolerant American elms for the nursery industry.

Technical Abstract:
The elms (the genus Ulmus) are one of the most important tree crops for the $4.7-billion per year nursery industry. The commercial importance of the genus centers on the American elm, Ulmus americana. Once decimated by Dutch Elm Disease, the recent introduction of cultivars resistant to the disease has led to a resurgence in horticultural use of the species, but this resurgence is currently based on very few cultivars, representing a perilously narrow genetic base. Early research concluded that Ulmus americana is a tetraploid species, which has been accepted dogma for 80 years. Flow cytometry was used to estimate ploidy level for trees from across the native range of U. americana in eastern North America. Two ploidy levels were found. Both diploid and tetraploid trees are common and widespread, and both ploidy levels occur together in some areas. Understanding the genetic relationships among the different ploidy levels, their geographic distribution and genetic relationship to one another will allow more efficient surveying for disease-resistant American elm germplasm, which will contribute towards developing novel, disease tolerant American elm cultivars for the urban forest.